1. Technical Field of the Invention
The present invention relates generally to the field of telecommunications and, more particularly, to signal processing in subscriber line modems.
2. Description of the Related Art
The rapid increase in use and popularity of the Internet has motivated research and development of systems directed to advanced communication of information between remotely located computers, particularly in effecting higher bit-rates using existing infrastructure. One type of technology arising from this development is referred to in the art as digital subscriber line (DSL). DSL refers generically to a public network technology that delivers relatively high bandwidth over conventional telephone copper wiring at limited distances. DSL has been further separated into several different categories of technologies according to particular expected data transfer rate, the type and length of medium over which data is communicated, and the schemes for encoding and decoding the communicated data. A DSL system can be considered as a pair of communicating modems, one of which is located at a client site, such as a home or office computer, and the other of which is located at a network control site, typically a telephone central office.
One type of DSL technology is referred to as Asymmetric Digital Subscriber Line (ADSL) and corresponds to the ANSI standard T1.43. ADSL technology encompasses communication according to Discrete Multitone (DMT) modulation and also includes frequency domain multiplexing (FDM), other modulation techniques are also known in the art. ADSL systems can communicate data over a single copper twisted pair at downstream (central office to remote client) rates on the order of 100 times that conventional voice band modem rates. For example, ADSL can utilize a downstream signal bandwidth of 25 kHz to 1100 kHz and an upstream signal bandwidth of 25 kHz to 138 kHz. Signal echo problems in this type of communication system are especially noticeable at the remote client modems, since its transmission bandwidth is within the bandwidth of its received traffic. Even in a FDM system, leakage of the upstream energy into the downstream band causes signal echo problems.
Because of the nature of ADSL communications, mixed signal circuitry is required in the implementation of ADSL modems, both for the central office and also for the remote clients in which both analog and digital signals are handled. Conventional ADSL modem designs include functions referred to as analog front ends in which operations such as digital-to-analog and analog-to-digital conversion, amplification/attenuation and filtering is performed. Because of the frequencies involved in ADSL technology, which can range from tens of kHz to MHz frequencies, and because of the large dynamic range required in order to accommodate the wide variations in length and schemes for subscriber loops, the amplification and filtering can be vary complex, particularly in an integrated circuit.
Analog amplification or attenuation is typically needed in the received path of most digital modems to fully utilize the available digital dynamic range. Typically, the gain adjustment depends on the particular channel conditions, therefore, automatic gain control is preferable. In cases where all the analog filtering is performed before all programmable gain attenuation/amplification (PGA) stage(s), the task of an automatic gain controller is greatly simplified as saturation conditions at the output of the filtering stages are readily observable in the digital domain. On the other hand, when the PGA and filtering stages are interleaved (typical in integrated analog front ends) the automatic gain controller problem is a difficult one since, in most environments, the intermediate points in the receive path are inaccessible.
Therefore, there is a need for an automatic gain control approach for digital subscriber line modems targeted to analog front end (AFE) designs with interleaved gain and filtering stages in which the intermediate points in the receive path are inaccessible.